The Fragile-X syndrome is one of the most common forms of mental retardation with a well-defined genetic etiology. The clinical management and educational guidance of affected individuals, as well as genetic counselling for family members depend critically on informed knowledge about the intellectual potential and psychological performance profiles of affected individuals. Although the syndrome has been described primarily in males, identified female carriers of the Fragile-X may show mild forms of developmental disability, but almost nothing is known at present about their intellectual potential and behavioral characteristics. The project will examine the behavioral features of institutionalized and non-institutionalized males with the Fragile-X site, and of obligate female carriers with and without the Fragile-X, as well as unaffected controls who either are or are not retarded, and who are selected specifically for each subgroup. All subjects will be examined by an extensive neuropsychological protocol that emphasizes component aspects of language function, motor coordination, and spatial abilities, as well as for patterns of social communication. Affected individuals are identified by currently accepted methods of karyotyping for the Fragile-X, and these will be complemented by more fined molecular linkage techniques once the latter are operational. Individuals with a confirmed Fragile-X, and genetic relatives as well as unrelated controls, will be examined by behavioral measures to test the level of overall intellectual potential and different profiles of performance deficits. Results from such studies will be compared between affected males who are or are not institutionalized, affected females and unaffected obligate female carriers. The purposes of the study are two-fold: to provide a detailed description of the behavioral characteristics in affected individuals; and to explore the potential of the syndrome as a prototype for investigations on the behavior genetics of mental retardation.